Irrigation systems that deliver water, often containing plant nutrients, pesticides and/or medications, to plants via networks of irrigation pipes are very well known. In some irrigation systems, external sprinklers, emitters or drippers, are connected to the irrigation pipes to divert water from the pipes and deliver the water to plants. In many such irrigation networks, water from the pipes is delivered to the plants by emitters or drippers that are installed on or “integrated” inside the irrigation pipes. For convenience, any of the various types of devices used in an irrigation system to divert water from an irrigation pipe in the system and deliver the diverted water to the plants is generically referred to as an emitter. Spacing between emitters, and emitter characteristics are often configured to respond to different irrigation needs of plants that the irrigation system is used to irrigate.
For a given configuration of irrigation pipes and emitters, quantities of water delivered by the irrigation system may be controlled by controlling any of various water flow control devices, such as water pumps, flow valves and check valves, and/or combinations of flow control devices known in the art. Flow control devices may operate to control water from a source that provides water to all of, or a portion of, irrigation pipes in an irrigation system or to control water from individual emitters in the irrigation system.
Israel Patent Application 177552 entitled “Irrigation Pipe” filed Aug. 17, 2006, the disclosure of which is incorporated herein by reference, describes an irrigation system having irrigation pipes comprising integrated emitters having different pressure thresholds at which they open to deliver water from the pipes. Which emitters open to deliver water, is controlled by changing pressure in the irrigation pipes. U.S. Pat. No. 5,113,888, “Pneumatic Moisture Sensitive Valve”, the disclosure of which is incorporated herein by reference, describes a spray device having its own valve that is opened and closed to control amounts of water that the device sprays on plants.
Various automatic and/or manual methods and systems are used to determine when and how much water to supply to plants irrigated by an irrigation system and to control water flow devices in the system accordingly. U.S. Pat. No. 5,113,888 noted above, controls the water flow valve in the spray device described in the patent responsive to soil moisture. The spray device comprises an element located in the soil that has pores, which are blocked when soil water moisture is above a predetermined amount and that are open when soil moisture is below a predetermined amount. When the pores are open, air is released from a chamber in the valve relieving pressure that keeps the flow valve closed to allow the valve to open and water to flow to and be sprayed from the spray device. U.S. Pat. No. 6,978,794, the disclosure of which is incorporated herein by reference, describes controlling an irrigation system responsive to soil moisture determined by at least one time domain reflectometry sensor (“TDRS”) located in the soil. The patent describes using multiple TDRS's at a different soil depth to provide measurements of soil moisture content. U.S. Pat. No. 6,314,340, the disclosure of which is incorporated herein by reference, describes controlling water responsive to diurnal high and low temperatures.
For many agricultural and scientific applications, soil water matric potential is used as a measure of soil moisture content and suitability of soil conditions for plant growth and irrigation systems are often controlled responsive to measurements of soil matric potential. Water matric potential, conventionally represented by “ψ”, is a measure of how strongly particulate soil matter attracts water to adhere to the particulate surfaces. The drier a soil, the stronger are the forces with which soil particles attract and hold water to their surfaces and the greater is the water matric potential. As matric potential of a soil increases, the more difficult it is for plants to extract water from the soil. When soil gets so dry that plants cannot extract water from the soil, plant transpiration stops and plants wilt.
Matric potential has units of pressure, is typically negative, and is conventionally measured using a tensiometer. A tensiometer usually comprises a porous material that is connected by an airtight seal to a sealed reservoir filled with water. The porous material is placed in contact with soil whose matric potential, and thereby moisture content, is to be determined and functions to couple the reservoir to the soil to allow water but not air to pass between the reservoir and soil. The forces that attract water to soil particles draw water through the porous material from the reservoir and generate a vacuum in the reservoir. The drier the soil, the greater are the forces that draw water from the reservoir through the porous material and the greater is the vacuum, i.e. the pressure of the vacuum decreases. As soil moisture increases, the forces that attract water to the soil particles decrease and water is drawn from the soil through the porous material into the reservoir and pressure of the vacuum increases. The vacuum increases (pressure decreases) or decreases (pressure increases) as water content of the soil respectively decreases or increases. A suitable pressure monitor is used to determine pressure of the vacuum and thereby provide a measure of the soil matric potential.
The porous material in a tensiometer is usually a ceramic and is often formed having a cuplike or test tube-like shape. However, U.S. Pat. No. 4,068,525, the disclosure of which is incorporated herein by reference, notes that the porous material “may be formed from any of a wide variety of materials, including ceramics, the only requirement being that the ‘bubbling pressure’, the pressure below which air will not pass through the wettened pores of the material, must be greater than normal atmospheric pressure, to prevent bubbles of air from entering the instrument”. It is noted that bubbling pressure is generally maintained only when the porous material is saturated with water.
Additionally, the porous material should provide good hydraulic contact between the soils and the water reservoir. The latter constraint with respect to soil contact generally requires that the porous material be in relatively intimate mechanical contact with soil particles. Whereas such contact can usually be provided by a surface of a ceramic, for coarse soils or gravels, such mechanical and resulting hydraulic contact can be difficult to obtain using a ceramic material. Gee et al, in an article entitled “A Wick Tensiometer to Measure Low Tensions in Coarse Soils”; Soil Sci. Soc. Am. J. 54:1498-1500 (1990) describes a tensiometer for use in coarse soils in which the porous material “is constructed from paper toweling or other comparable wicking material rolled tightly into a cylinder (.about.0.7 cm in diameter and .about.7 cm long).” The authors note that the tightly rolled wicking material when wetted was pressure tested for suitable bubbling pressure.
U.S. Pat. No. 5,156,179, the disclosure of which is incorporated herein by reference, describes an irrigation system that is controlled using a tensiometer responsive to water matric potential. The system comprises a “flow controller device” that includes a valve assembly connected with the tensiometer to “provide automatic control of flow of water for irrigation”. Changes in pressure in the tensiometer move a piston in the valve to provide “variable control of the rate of flow” through the valve assembly “according to the matric tension of the soil for water”.